QBARS - v17n1 Dwarf Rhododendrons

Dwarf Rhododendrons
by Virginia Jefferis
From an illustrated talk delivered at Media, Pa. to the Philadelphia Chapter ARS

Rock garden with dwarf rhododendrons
Fig. 2.  The rock garden with dwarf rhododendrons
Jefferis photo

When Dr. Franklin West, our Program Chairman, asked for this talk he specified one on lepidotes and really caught me unprepared. I wasn't even too sure just what constituted a lepidote. We first became interested in dwarf rhododendrons because of their desirability for a small shady rock garden, and for the past three years I've been collecting the plants in a kind of un-intellectual way that gave me no idea how they fitted into the lepidote vs. elepidote classification. So the first thing I had to do was to list all our dwarf rhododendrons in their series and then find out whether those series were lepidote or elepidote. 1

1 See Cox, E. H. M., and Cox, P.A., Modern Rhododendrons, Thomas Nelson and Sons Ltd., 1956; and Leach, David G., Rhododendrons of the World, Charles Scribner's Sons, 1961

By the time that was done, and it was obvious that our dwarfs were about evenly divided between lepidotes and elepidotes, I began to be curious about the actual difference between them and whether there was any easy way to classify a rhododendron by casual observation. The answer seems to be: only within limitations. There are certain generalities that can be made about lepidotes and elepidotes which make it possible to guess intelligently about the classification of a rhododendron, but the exceptions will confuse you if you rely on casual observation alone, In general lepidotes are small to medium in size and elepidotes range from large to tree-like. However, R. minus , which is a lepidote, grows 20'-30' feet tall, while one of the most attractive elepidotes R. forrestii var. repens creeps along the ground, For the most part lepidotes have small leaves and flowers, and elepidotes have the broader leaves and larger flowers of the average garden hybrids, But this obviously doesn't cover elepidote dwarfs which have relatively small leaves and flowers. In the final analysis there is only one decisive difference between lepidotes and elepidotes: the lepidotes have scales, the elepidotes do not, Both may have papillae, another type of trichome or leaf covering; both may have hairs (in fact elepidotes usually do); but only the lepidotes have scales.
It isn't pertinent here to expand the subject of the rhododendron leaf and its trichomes except to say that all three types of trichomes serve the same purpose of regulating transpiration. They aid the plant in giving off the large quantities of water which it takes in during the rainy season, and prevent it from becoming too dried out during windy and dormant seasons. 2

2 Cowan, J. II., The Rhododendron Leaf, Oliver and Boyd, 1950

There is no sure way of determining the presence or absence of scales with the naked eye, but after examining several dwarf rhododendron leaves under a microscope I got so carried away with this phase of rhododendron study that I had some macro-photographs made to show a little of what I found out.
If you were to examine the back of a rhododendron leaf with a hand lens of 10x or more you would discover some of the plate-like discs and know you were looking at a lepidote. This is R. ferrugineum which is so heavily covered with scales that they overlap and make the leaf appear brown on the underside even to the naked eye. Of course the more scales or hairs there are the more the leaf is protected from transpiration, and you can see that R. ferrugineum is well suited to its habitat 4,000 feet up in the Alps. It also seems to adapt well to our area. This is a cross section of a leaf of R. carolinianum , another lepidote, showing how scales are like saucers on a stem, with the stem fitting into a depression in the under surface of the leaf and the saucer providing a protective layer just above the surface. This slide shows a cross section of an elepidote, R. yakushimanum , illustrating the brown furry indumentum made up of two of the several types of hairs and also providing excellent protection for the leaf. This slide is the under surface of a leaf of R. hemitrichotum , a lepidote with all three types of trichomes-scales, hairs and papillae.
While some of the dwarf rhododendrons seem difficult to cultivate in our area, and some would be of interest only to collectors, most of them are reasonably tractable, and there are several which rate among the most beautiful of all flowering evergreens. As a group they extend the season of bloom considerably, since the lepidotes are at their best about a week ahead of the early azaleas and the dwarf elepidotes continue to bloom when most of the Kurumes have faded. They range through all the flower colors common to the other rhododendrons and evergreen azaleas. They also add many more plants - which bloom in shades of blue, blue-violet and purple. These are particularly noticeable in contrast to pinks and whites of dogwoods and azaleas, or to the bright colors of many of the early spring bulbs.
They provide endless foliage variety and interest throughout the year. For example, such dwarfs as R. scintillans and R. fimbriatum have delicate leaves almost as narrow as yew needles; R. leucaspis leaves are oval, two inches long and an inch or more wide. The foliage of R. impeditum is noticeably blue-green in color. R. sanguineum is black-green. R. microleucum is yellow-green. Some leaves, like those of R. degronianum and R. racemosum have an under surface decidedly different in color from the upper surface. Thoughtful placement of the many shapes and colors in dwarf rhododendron foliage can create very pleasing contrasts.
Winter produces a whole new color effect using the very same plants. R. fastigiatum which is a lovely blue green in summer is a no less beautiful maroon in winter. R. 'Blue Diamond' becomes a warm dark red and R. 'Bluebird' turns a bright red reminiscent of autumn. The buds of next spring's bloom are yellow-green against the dark green foliage of R. forrestii var. repens . The buds of R. 'Cilipenense' are red against bright green leaves. R. 'Hockessin's' buds are pink against the dark red color of its winter foliage.
A not so pretty picture is presented in winter by a few dwarfs such as R. uniflorum , R. hippophaeoides and R. scintillans . Their leaves shrivel with the cold till the plants look completely dead. Their revival with the first-warm days of spring is one of the unexpected rewards of growing dwarf rhododendrons.
In the search for plants which will be scaled to today's small properties the dwarf rhododendrons should be in increasing demand. Such plants as R. carolinianum , R. 'Windbeam' and R. makinoi grow large enough to make foundation plantings in proportion to modern ranch houses. Miniatures like R. impeditum , R. nitens , R. pemakoense , R. radicans and R. keleticum will never outgrow even a tiny rock garden. Medium growers, R. 'Blue Diamond,' R. hemitrichotum , R. 'Jock,' R. 'Augfast' and R. 'Wyanoki' are good plants for the foreground of a border. Many which start off as small plants in our rock garden are transferred to the border in a few years; and some which started in the border have been moved to the rock garden in order to provide a setting more in scale with their size or conditions more to their liking.
This brings up the subject of culture. Complete success has certainly not crowned all my efforts, but three or four years of experimenting have disclosed a few general truths which pertain to growing dwarfs well in this area. Almost all of them are plants native to extremely high elevations or are epiphytic (R. moupinense) . In either case sharp drainage and an airy medium are even more vital to their health than is true with the average hybrid rhododendron. This can he achieved in several ways, but in our location, where the soil is heavy, we have found that raising the beds and using large quantities of sand are two essentials. This past year I saved several ailing dwarfs by setting them on top of the level from which they were dug and putting a 50-50 sand and peat moss mixture around the root halls. It was amazing to watch the green color return to a plant of R. 'Sapphire' treated this way. The one exception seems to be R. hippophaeoides which didn't do well until it was planted in a pocket lined with polyethylene and filled with sphagnum moss. Apparently this is a bog loving plant.
Hardly less important to the dwarfs in general is keeping them cool during our hot summers. Shade, of course, is one way; and shade from the strong winter sun is important too. White pines are particularly good with rhododendrons: A pine tree only a few inches high will do a good job of shading a small rhododendron and will keep ahead of it in growth. Another way of keeping plants cool is to provide air circulation and air drainage. Plants which dislike a bed at lawn level will thrive on a slope or an elevation. This same elevation will also keep the plants warmer in winter since the cold air will settle to the lowest part of the garden.
The last suggestion I can make for keeping the dwarfs happy is this: having provided them with sharp drainage and a light medium they will be benefited by an occasional cooling sprinkle during hot dry spells.
Getting plants off to a good start is important. A method which we have found invaluable is to acquire the plants in the fall and keep them over the winter in the "shanty." This shanty is a homemade redwood frame covered tightly on all sides with 4 mill polyethylene and unheated. It is placed on the southeast side of our stone garage; put together with screws; and can be taken out of the ground easily by hand if it ever outlives its usefulness. The frame is 9 feet long, 42 inches wide, 24 inches high in front and 30 inches high in back. The top is made in three removable sections. We have used various planting media with equal success: half peat, half perlite; half peat, half sand; 100% peat.
In each case the mixture goes on top of a generous drainage layer of sand or pebbles; DDT, a fungicide, and plant foods are added, the latter being ammonium sulphate, potassium sulphate and super-phosphate. But these are not necessary as we have had success with and without the addition of pesticides, fungicides and fertilizers. The medium is watered either by one or two heavy rains or by hose. Once the plants are put in the shanty (after frost occurs and fall has set in-late November) and the top fastened in place, the whole setup can be forgotten till spring. The inside temperature never seems to get too high, and sufficient moisture comes through the soil. On an Airguide Humidity Indicator the relative humidity stays consistently over 90% Beads of moisture are present at all times on the inside of the top and sides.
Although plants in the shanty are not off-cycled as they sometimes are in a greenhouse, nevertheless they seem to make considerable root growth under the conditions of still air and high humidity. This enables them to overcome the shock of shipping and to establish readily when they are put into their permanent location in the spring.
I could group the dwarfs we are growing by color, or list them in order of ease of cultivation or preference, but it appears more logical to arrange them in terms of lepidote or elepidote and according to their series, including with each species plant the hybrids of it. The following are plants which are on our property at the present time, with some indication of how long we have had them. Complete descriptions are not included because they can be obtained from many sources, but I hope the list of names will encourage you to try something new in your own gardens.

Series Remarks
sargentianum We have 'Maricee' A. E. which is a white blooming seedling. Very dwarf. Drainage and shade important
leucaspis* Very easy to grow. Consider it worth a place in the garden for foliage alone. Flower buds have always been frost killed here.
'Seattle Springtime' ( leucaspis x mucronulatum )
'Snow Lady' ( leucaspis x ciliatum ) Same remark as leucaspis .
campylogynum We have 'Patricia', a dark purple seedling. Very dwarf and easy to grow if given good drainage.
Carolinianum Three series members, all native to the eastern U.S.; hardy and easy.
carolinianum ** One of the most beautiful evergreens for foundation plantings. Blooms profusely even in total shade. Very useful for hybridizing with other lepidotes.
'Windbeam'** ( carolinianum x racemosum ) x ? To five feet in 20 years. Very floriferous; apricot buds open to white and fade to soft pink.
'Wyanokie'** Same cross as 'Windbeam' but more dwarf in habit and always white. Two feet tall in 10 years.
'Ramapo'** ( carolinianum x fastigiatum ) Lovely blue-violet blooms on compact, hardy bush. Two feet in 10 years.
ferrugineum Known as Alpine Rose. Foliage deep shiny green all year.  Doesn't appear to be difficult if given some shade in the summer.
hirsutum Oldest rhododendron in cultivation. Slightly smaller than ferrugineum and not as green. Hasn't done as well for me as R. ferrugineum .
kotschyi Very compact growth: attractive dark green foliage. Easy.
Lapponicum There are 52 species: all very hardy and most of them small plants attractive all the year round, with small leaves and flowers.
chryseum Gray-green aromatic foliage. Yellow flowers. Often blooms in fall. Sensitive to heat and poor drainage.
fastigiatum F. C. C.* Deep purple.
'Yellow Hammer' ( sulfureum x flavidum ) Upright grower, narrow leaves, yellow flowers.
hippophaeoides Aromatic foliage and lovely lavender-blue flowers. Likes more moisture than most dwarfs. Variety 'Haba Shan' considered excellent.
impeditum * One of the most dwarf, making a dense bush only about one foot tall.
intricatum * Free flowering blue.
microleucum * 15"-18" in 10 years: mauve to white flowers.
russatum Have tried this several times but haven't yet been able to establish it. F. C. C. form has deep purple flowers.
scintillans F. C. C. Deep purplish blue flowers, rather sprawling habit.
Moupinense Mr. D. W. James calls this the queen of all alpine species though I suspect that it blooms so early that in this area its buds may be frost bitten. Apparently sensitive to heat and very sensitive to poor drainage. Beautiful foliage.
'Cilpinense' ( ciliatum x moupinense )
'Seta' ( spinuliferum x moupinense )
'Tessa' ('Praecox' x moupinense ) I like the members of this series because they are compact and bloom readily when young.
calostrotum Light blue-green foliage, wide pinkish purple flowers.
'Cutie' P.A.* ( calostrotum ? x ?) Phlox purple, very compact and bushy. Foliage reddish brown in winter.
keleticum Semi-prostrate to 2 feet wide in 10 years. Flowers crimson purple and very large in proportion to the plant. Flowers when young. Blooms in June.
radicans * There is a prostrate form and a taller form, both attractive, very both blooming when very small with relatively large blooms.
Scabrifolium One of my favorites. Blooms in April with pink dainty pale flowers. Has very graceful habit of growth. Foliage hairy and a lovely silvery green all year.
hemitrichotum *
mollicomum *
spiciferum* Widely spreading graceful shrub with deep pink flowers. Seems tolerant to a variety of conditions.
lepidostylum Said to be easy but I haven't yet succeeded with it, apparently because the location was too hot or the soil not drained well enough. The dramatic effect of its blue-green foliage encourages me to keep trying.
Triflorum More than 50 species in this series from which come some of the easiest and most desirable of the lepidote dwarfs for this area.
augustinii I believe only Mr. Gable's hardy form has a chance in this area. It's a paler blue than the named varieties but very beautiful. Many of the augustinii hybrids are more dwarf, hardier and very easy to grow.
'Augfast'* ( augustinii x fastigiatum ) Three feet in 10 years; violet blue. Will stand considerable sun.
'Bluebird'* ( intricatum x augustinii ) Two feet in 10 years. Medium blue. Will stand considerable sun.
'Blue Diamond'** ( intrifast x augustinii ) Four feet in 10 years, compact. Very deep blue. Heat and sun tolerant.
'Blue Tit'* ( impeditum x augustinii ) Two feet in 10 years. Light blue.
'Rustic Maid' ('Blue Diamond' x russatum )
'Sapphire'* ('Blue Tit' x impeditum ) Small with possibly the clearest, brightest blue flowers of all rhododendrons. 18 inches in10 years.
davidsonianum Not exactly a dwarf - 4 feet to 5 feet in 10 years. There are several forms. Ours is 'Serenade.'
hanceanum var. nanum Apparently somewhat heat tolerant.
keiskei * Two forms, a tall growing rather leggy one and a dwarf form which is very desirable. Early blooms of pale yellow.
Guyencourt hybrids ( keiskei x pubescens ) As a group very hardy and desirable in this area. Color variations are:
'Brandywine' - Rose
'Chesapeake' - Apricot fading to white.
'Delaware' - Apricot fading to white
'Hockessin'* - Apricot fading to white
'Lenape' - Pale yellow
Montchanin' - White
'Mary Fleming' ( racemosum x keiskei ) x keiskei - Salmon yellow
oreotrephes * Four to five feet in 10 years; oval bluish-green leaves. Mauve pink flowers in miniature trusses.
pemakoense Have not been successful with previous attempts with this plant but think it may succeed if given more sand and winter shade.
uniflorum * Very dwarf. Blooms when very small with mauve pink flowers.
racemosum * Recommended without reservation. Highly adaptable. Leaves dark green all year. Flowers range from almost white to dark pink. Interesting red wood.
'Pioneer' ( racemosum x mucronulatum ) x ? - Bright rose pink and early.
'Puck' ( racemosum x spiciferum )
'Racil'* ( racemosum x ciliatum ) Dainty shell pink bloom. Very floriferous. Inclined to be a little bare of foliage.
'Twinkles' ( racemosum x spiciferum )

R. forrestii var. repens encased in ice.
Fig. 3. R. forrestii var. repens encased in ice.
Jefferis photo

Series Remarks
camtschaticum Deciduous
Neriiflorum Contains some of the most beautiful dwarfs, small to medium in size, usually having waxy red flowers.
apodectum Orange-red tubular flowers.
forrestii var. repens * Very desirable prostrate plant. Perfectly hardy but must have sharp drainage. Hybrids are supposed to bloom more readily and be less difficult to grow.
'Carmen' ( didymum x forrestii var. repens ) Deep red tubular bloom.
'Elizabeth' ( forrestii var. repens x griersonianum ) Bright red. Some forms seem to be hardier than others but it's a question whether any of them will survive long outdoors in our area. Ours has withstood +5 F. without protection.
'Little Joe' ( forrestii var. repens x 'May Day') Bright dark red.
'Ostfriesland' ('Madame de Bruin' x forrestii var. repens ) Quite sensational with large red bud against large bright green leaves.  Plant habit very compact.
'Yeoman'* ( forrestii var. repens x 'Choremia') Turkey red. Prostrate. May not be completely hardy.
gymnocarpum A.M.
haematodes Should survive here but I have had trouble with it. Apparently needs 100% shade winter and summer.
'Vega'* ('Fabia' x haematodes ) Seems to have lost wood every winter so far. Perhaps it isn't really hardy enough.
sanguineum Rock No. 6. Needs full shade.
sperabile A.M.
degronianum To five feet but only 1 foot in 10 years. Dark green convex leaves with fawn indumentum.
makinoi Not exactly a dwarf but slow growing to 6 feet. Long narrow leaves with tan indumentum.
yakushimanum F.C.C. This form has dark green, shiny convex leaves with orange brown indumentum. Very short new growth rapidly turns palest fawn. Have two Serbin forms which develop silvery new growth with some whitish coloration on leaves.
williamsianum One of the most useful species for hybridizing. Beautiful round shiny leaves, copper red new growth and pink flowers.
'Adrastia' ( williamsianum x neriiflorum ) Grows low and spreading. Blooms when young with deep pink bell-like flowers.
'Arthur Ivens' ( williamsianum x houlstonii )
'Bow Bells'* ('Corona' x williamsianum ) Generally considered one of the 10 best rhododendrons. Quite hardy and will stand considerable sun.
'Cowslip' ( williamsianum x wardii )
'Dormouse' (Dawn's Delight' x williamsianum )
'Humming Bird'* ( haematodes x williamsianum ) Very compact. Pink bloom.
'James Barto'* P.A. ( williamsianum x orbiculare ) Two feet in 10 years. Large leaves and flowers for such a small plant. Very hardy, even to its buds. Flowers pink.
'Jock'** ( williamsianum x griersonianum ) One of the best low spreading growers. Very hardy and very heat and sun tolerant, though it does equally well in shade. Dark pink bloom.
'Moonstone'* ( campylocarpum x williamsianum ) Two feet in10 years. Cream colored flowers. One of the finest dwarfs and quite hardy.
'Thomwilliams' ( thomsonii x williamsianum )
'Treasure'** ( forrestii var. repens x williamsianum ) Very nice dwarf habit and rounded green leaves. I have heard that there are two forms of this plant-ours blooms white but all the books say Treasure is dark pink.
'Whimsey' ( souliei x 'Bow Bells') This plant was killed back almost to the ground the first year. Then I put a dwarf hinoki cypress on the windy side and it has done well since.

* These plants were in our garden at least as far back as the hard winter of 1960-61, some of them longer.
** These are especially desirable, even for those who are not necessarily collectors of dwarf rhododendrons. The rest of the plants we have not had long enough to be able to speak about them with any authority.
For people in this area who are interested in making a collection of dwarfs, in addition to those with ** above. I can at present recommend the following:

campylogynum kotschyi 'Cutie'
fastigiatum F.C.C. microleucum 'Moonstone'
ferrugineum radicans 'Pioneer'
forrestii var. repens uniflorum 'Racil'
hemitrichotum yakushimanum 'Sapphire'
hippophaeoides 'Augfast' 'Treasure'
impeditum 'Blue Bird'
keleticum 'Blue Tit'